Metal mold manufacturing method, metal mold, optical element, and optical element manufacturing method

ABSTRACT

A material of a cemented carbide having tungsten carbide as a major ingredient is cut and polished into a shape, a silicon coating as a protecting layer is applied thereto by the plasma CVD method, and a chromium nitride coating with a thickness of 1 μm is applied thereto to form a preliminary shape metal mold. The shape of the formed preliminary shape metal mold or the shape of a sample formed with the preliminary shape metal mold is measured, and from the manufacture error thereof, the shape correction amount of a preliminary shape metal mold  30  for use as a drag is obtained. Based on the obtained shape correction amount, correction by etching is performed on a drag molding surface  301  of the drag preliminary shape metal mold  30  so that a predetermined precision is obtained, thereby completing the metal mold. An optical element or a part is manufactured by use of the metal mold.

[0001] This application is based on application No. 2001-144661 filed inJapan, the content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a technology to manufacture asmall-size optical element and the like with high precision.

[0004] 2. Description of the Related Art

[0005] In the fields of optical pickups, optical communications and thelike, elements such as high-precision and small-size lenses and mirrorsare required as information capacity increases. As a method ofmanufacturing such elements, molding to transfer the shape to theoptical base material by use of a metal mold has been proposed.

[0006] For example, as a lens manufacturing method using molding, amethod has been proposed such that after a prototype of the metal moldis obtained by cutting or grinding the metal base material, the shape ofthe metal mold itself or the shape of a lens molded with the metal moldis measured, and the manufacture error of the metal mold is corrected bymachining in accordance with the result of the measurement.

[0007] As a microlens manufacturing method using lithography, forexample, a method using isotropic etching has been proposed. Further, amethod using reflow has been proposed.

[0008] Metal molds for molding are formed by machining such aspolishing, grinding and cutting, and the machining precision is limitedby the tip curvature of the cutting tool for machining and themechanical positioning precision of the cutting tool for the metal mold.Because of this, it is difficult to machine or correct metal molds withhigh precision in manufacturing tiny optical elements with a diameter ofnot more than 1 mm.

[0009] For example, a high-numerical-aperture (NA) reflective-typeelement which is an optical element requires a profile irregularity notless than three times stricter than that of a refractive-type element.Uses requiring a high-precision wave front with a wave front distortionof approximately 0.1 of the wavelength requires a surface machiningprecision of as high as not more than 0.05 μm. However, with the priorart, because of the above-mentioned reasons, it is difficult to obtainmetal molds with which such high-precision elements can be formed.

[0010] According to the method using reflow, the obtained shapes arelimited because of the nature of the method that a heat deformation dueto surface tension is used. In addition, it is difficult to obtainhigh-precision surfaces with stability.

[0011] When etching is used, machining can be performed with highprecision. However, since the depth of one etching is comparativelysmall, if all the process of obtaining a final three-dimensional shapeout of a base material is performed only by etching, it is necessary torepeat etching a multiplicity of times, and manufacture efficiency islow.

[0012] These problems arise not only with optical elements but also withvarious manufacture objects such as parts requiring high shapeprecision.

SUMMARY OF THE INVENTION

[0013] The present invention is made in view of the above-mentionedproblems, and an object thereof is to efficiently form manufactureobjects such as optical elements with high precision.

[0014] One aspect of the present invention is a method of manufacturinga metal mold for molding an object, comprising: preparing a preliminaryshape metal mold having a shape approximate to an outside shape of theobject to be molded; and processing for obtaining a molding surface byselectively removing a surficial part of the preliminary shape metalmold by etching.

[0015] Another aspect of the present invention is a metal mold formolding an object, comprising: a first portion of molding surfaceobtained by a preliminary manufacturing stage; and a second portion ofmolding surface obtained by selectively removing a surficial partobtained by the preliminary manufacturing stage by etching in accordancewith a predetermined shape correction amount.

[0016] Still another aspect of the present invention is a method ofmanufacturing an optical element, comprising: preparing a preform havinga shape approximate to an outside shape of the optical element; andselectively etching a surface shape of the preform for obtaining theoptical element.

[0017] Further aspect of the present invention is an optical elementcomprises: a first surface portion obtained by a preliminarymanufacturing stage having a shape approximately corresponding to apredetermined optical function; and a second surface portion obtained byselectively removing a surficial part obtained by the preliminarymanufacturing stage by etching.

[0018] These and other objects, advantages and features of the inventionwill become apparent from the following description thereof taken inconjunction with the accompanying drawings, which illustrate specificembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In the following description, like parts are designated by likereference numbers throughout the several drawings.

[0020]FIG. 1 showing a first embodiment of the present invention shows ahigh-NA reflective-type lens;

[0021]FIG. 2 is a flowchart showing a process of manufacturing thehigh-NA reflective-type lens;

[0022]FIG. 3 shows a manner of manufacturing a sample by use of apreliminary shape metal mold for use as a cope and a preliminary shapemetal mold for use as a drag;

[0023]FIG. 4 shows a manner of manufacturing the high-NA reflective-typelens by molding;

[0024]FIG. 5 shows a manner of measuring the manufacture error of thehigh-NA reflective-type lens with a Mach-Zehnder interferometer;

[0025]FIG. 6 shows an example of interference fringes obtained by theMach-Zehnder interferometer;

[0026]FIG. 7 shows a ray tracing technique using a calculator;

[0027]FIG. 8 shows a relationship between a corrected molding surfaceand an ideal molding surface;

[0028]FIG. 9 shows an error evaluation of the corrected molding surface;

[0029]FIGS. 10A to 10F show a manner of correcting the preliminary shapemetal mold for use as a drag by etching;

[0030]FIG. 11 shows a manner of measuring the manufacture error of ametal mold of a third embodiment with a laser interferometer;

[0031]FIG. 12 shows a ray tracing technique using a calculator;

[0032]FIG. 13 shows a microlens of a fourth embodiment;

[0033]FIG. 14 is a flowchart showing a process of manufacturing apreform of the microlens;

[0034]FIGS. 15A to 15D show a manner of manufacturing the preform of themicrolens by reflow;

[0035]FIG. 16 is a flowchart showing a process of manufacturing themicrolens from the preform of the microlens; and

[0036]FIGS. 17A to 17F show a manner of correcting the preform of themicrolens by etching.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Preferred embodiments of the present invention will be describedin detail with reference to the attached drawings.

[0038] <1. First Embodiment>

[0039] In a first embodiment, a preliminary shape metal mold formed by amanufacturing process such as cutting and grinding is corrected byetching, and a high-NA reflective-type lens is formed with the correctedmetal mold. The NA means “numerical aperture” which is the product ofthe sine of the angle between the light ray and the optical axis and therefractive index of the medium.

[0040]FIG. 1 shows a high-NA reflective-type lens 1 formed in the firstembodiment. The high-NA reflective-type lens 1 has an NA of 1.3 and adiameter of several millimeters, and is used, for example, as a solidimmersion lens generating near-field light for optical pickups foroptical memories or in optical communications. A light ray incidentthrough a first aperture 11 of the lens 1 is reflected at a secondreflecting surface 14, and undergoes a condensing function at a firstreflecting surface 13 to be condensed to a second aperture 12. Asdescribed above, optical elements having a plurality of reflectingsurfaces require an accuracy of a wave front distortion of as high asapproximately 0.1 of the wavelength of the light used. For example, whenthe refractive index n of the optical member in the lens 1 is 1.6 andthe wavelength λ of the light used is 0.4 μm, considering the fact thatthe error of the reflecting surface exerts a twofold influence on theoptical axis, the maximum permissible error d is 0.0125 μm from theexpression 1.

d×n×2=0.1×λ  (1)

[0041] In the present embodiment, a metal mold and the lens 1 are formedso that the maximum error is not more than d by the following method:

[0042]FIG. 2 is a flowchart showing a process of manufacturing the lens1 in the first embodiment. FIG. 3 shows a manner of manufacturing asample prototype 100A by use of a preliminary shape metal mold 20 foruse as a cope and a preliminary shape metal mold 30 for use as a drag(these will be abbreviated together as preliminary shape metal molds 20and 30) which are a prototype of a metal mold for forming the lens 1 inthe present embodiment. FIG. 4 shows a manner of manufacturing the lens1 by molding by use of the corrected metal mold. With reference to FIGS.2 to 4, the manufacturing method of the present embodiment will bedescribed.

[0043] First, the preliminary shape metal molds 20 and 30 shown in FIG.3 to be the prototypes of a cope 2 and a drag 3 shown in FIG. 4 areformed (step S11). The cope 2 is a metal mold for forming the shape ofmainly a first reflecting surface 13 of the lens 1 with a cope moldingsurface 201 thereof. The drag 3 is a metal mold for forming the shape ofmainly a second reflecting surface 14 of the lens 1 with a drag moldingsurface 301 thereof.

[0044] The preliminary shape metal mold 30 to become the drag 3 later isformed in the following manner: The upper surface (a drag preliminarymolding surface 300) of a base material of a cemented carbide havingtungsten carbide as a major ingredient is cut and polished into a plane,a silicon coating as a protecting layer is applied thereto by the plasmaChemical Vapor Deposition (CVD) method, and a chromium nitride coatingwith a thickness of 1 μm is further applied thereto. The reason why thethickness of the chromium nitride layer is 1 μm is that since themanufacture error amounts of metal molds are generally smaller than 1μm, it is sufficient to presume the amount of shape correction byetching in the processing step described later to be not more than 1 μm.The preliminary shape metal mold 20 to become the cope 2 later is formedby cutting and polishing a base material of the cemented carbide into adesired aspherical concave surface (a cope preliminary molding surface200) and applying a silicon coating as a protecting layer and a chromiumnitride coating thereto.

[0045] By this method, preliminary shape metal molds having a shapeapproximate to the outside shape of the lens 1 can be prepared.

[0046] The method of manufacturing the preliminary shape metal molds 20and 30 is not limited to the above-described example. That is, it isdesirable that the following conditions be satisfied: the base materialcan be polished into a smooth specular surface with no pore; oxidationresistance at high temperatures is high; there is no change in structureand the like; the surface quality is maintained; the ingredients of theproduct neither fuse nor react with other materials; the product iseasily parted from the metal molds; and the hardness and the strengthare high at high temperatures, and as long as these conditions aresatisfied, the preliminary shape metal molds 20 and 30 may bemanufactured out of a different material or by a different method.

[0047] Then, using the preliminary shape metal molds 20 and 30, thesample prototype 100A of the high-NA reflective-type lens 1 is formed bymolding shown in FIG. 3 (step S12). A reflective coating is selectivelyapplied to a surface of the sample prototype 100A to obtain a sample 100(see FIG. 5) having a first aperture 11, a second aperture 12, a firstreflecting surface 13 and a second reflecting surface 14 like thehigh-NA reflective-type lens 1 shown in FIG. 1.

[0048] After the sample 100 is manufactured, the transmission wave frontof the sample 100 is measured with a measuring device to obtain theshape correction amount of the preliminary shape metal molds (step S13).FIG. 5 shows an out line of a Mach-Zehnder interferometer 4 which is themeasuring device for measuring the manufacture error of the formedsample 100. The Mach-Zehnder interferometer 4 shown in FIG. 5 comprises:a laser generator 41 generating a laser beam for the measurement; a beamsplitter 42 splitting the incident laser beam into reflected light 400and transmitted light 401; a mirror 43 further reflecting the reflectedlight 400; a microscope objective 44 directing the transmitted light 401to the first aperture 11 of the sample 100; a hemispherical mirror 45condensing the transmitted light 401 exiting through the second aperture12 of the sample 100, again to the second aperture 12; a taking lens 46for taking the interference fringes between the reflected light 400 andthe transmitted light 401; and a CCD 47.

[0049] If a virtual sample having no manufacture error and formed tohave the exact shape as designed is measured here, an image with nointerference fringes or with interference fringes like straight linesarranged in parallel will be obtained on the CCD 47. However, since thesample 100 generally has a manufacture error, an image with curvedinterference fringes is obtained on the CCD 47.

[0050]FIG. 6 shows an example of the interference fringes observed dueto the interference action of the Mach-Zehnder interferometer 4 shown inFIG. 5. A light ray 402 shown in FIG. 6 is, of the laser beam from thelaser generator 41 shown in FIG. 5, a light ray incident on a point P onthe second reflecting surface 14 of the sample 100, and part of thelight ray 402 is reflected at the beam splitter 42 and is furtherreflected at the mirror 43 to reach the CCD 47.

[0051] Of the light ray 402, the light transmitted by the beam splitter42 is condensed to the first aperture 11 of the sample 100 by themicroscope objective 44, and is reflected at the point P on the secondreflecting surface 14. Then, the light undergoes a condensing functionat a point Q on the first reflecting surface 13 to be condensed to thesecond aperture 12. The light condensed to the second aperture 12 isreflected at the hemispherical mirror, follows the same optical pathagain, and is reflected at the beam splitter 42 to reach the CCD 47. Thetwo light rays obtained from different optical paths interfere with eachother to form a plurality of interference fringes on the CCD 47. Aninterference fringe 403 represents one of the interference fringes. Dataof the observed interference fringe 403 and the like is input to acalculator or computer 48, and used for the processing described later.When a high-NA reflective-type lens with an NA of higher than 1 ismeasured, by bringing the hemispherical mirror 45 and the high-NAreflective-type lens into contact with each other or filling the gaptherebetween with an oil of a high refractive index, even such a lenscan be measured by the method shown here.

[0052] The shape of the actual transmission wave front of the sample 100is easily measured from the image of the interference fringe 403 thusobtained on the CCD 47. For example, fringe scanning to measure therelative phase of each point on the interference fringe 403 by movingthe mirror 43 in the direction of the optical axis is used.

[0053] Then, the manufacture error of the second reflecting surface 14of the sample 100 is obtained by use of a ray tracing technique. FIG. 7conceptually shows the ray tracing technique. A plane 501 is a planesupplied to the calculator 48 on the assumption that the secondreflecting surface 14 of the sample 100 is a horizontal plane. Further,the light ray 402 is supplied to the plane 501 and the position of thepoint P at which the light ray 402 is reflected on the plane 501 isshifted downward in FIG. 7 to obtain a point S where the actuallymeasured shape of the transmission wave front is obtained.

[0054] By doing this, the shape error of the transmission wave front isobtained as the distance between the points P and S. This operation isperformed for all the points P on the plane 501 and all thecorresponding points S are obtained to thereby obtain a plane 502. Thatis, the overall correction amount of the second reflecting surface 14 isobtained as the difference between the planes 501 and 502.

[0055] In the actual manufacture of optical elements having a pluralityof optical surfaces, a manufacture error occurs in both the firstreflecting surface 13 and the second reflecting surface 14. However,under a condition where the directions of the light rays incident on thereflecting surfaces are substantially the same (only one light ray isincident on each point on each reflecting surface) like in the presentembodiment, by correcting only the drag 3 for forming the shape of thesecond reflecting surface 14 on the assumption that the secondreflecting surface 14 includes all the manufacture errors, the errors ofa plurality of optical reflecting surfaces (in the present embodiment,the first reflecting surface 13 and the second reflecting surface 14)can be equivalently corrected all together, whereby the transmissionwave front of the formed high-NA reflective-type lens 1 can becorrected.

[0056] Since the light to be used is used for measuring the manufactureerror of the optical element in this manner, more practical measurementcan be performed. Assuming now that the maximum correction amount δ inthe present embodiment is 0.2 μm, a method of correcting this by etchingwill be described.

[0057] First, in performing etching, the minimum etching amount isobtained. FIG. 8 shows a corrected molding surface 503 having undergoneetching together with an ideal molding surface 504. The ideal moldingsurface 504 is a surface providing the design optical performance of themanufactured high-NA reflective-type lens 1.

[0058] When correction is performed by etching, the corrected moldingsurface 503 is stepped as shown in FIG. 8. When the minimum etchingamount in the etching (the size of the smallest step of the correctedmolding surface 503 shown in FIG. 8) is a minimum etching amount h, themaximum error d between the corrected molding surface 503 and the idealmolding surface 504 is half the minimum etching amount h. Since themaximum permissible error d is 0.0125 μm, the minimum etching amount his obtained as 0.025 μm.

[0059]FIG. 9 shows, together with the ideal molding surface 504, anestimated shape of the corrected molding surface 503 corrected so thatetching is not performed more deeply than the ideal molding surface 504at all times in etching. Since the minimum etching amount h is 0.025 μm,the corrected molding surface 503 is formed so as to be shifted from theideal molding surface 504 by 0.025 μm at the maximum.

[0060] For the precision of an optical element like the high-NAreflective-type lens 1, since the influence of the shape error of thereflecting surface is sufficiently large compared to that of theposition error (position error in the vertical direction in FIG. 9) ofthe reflecting surface, the error evaluation may be performed byregarding a plane 505 as the new ideal molding surface. The plane 505 isthe ideal molding surface 504 parallelly shifted by 0.0125 μm in thevertical direction in FIG. 9. Since the maximum error d between thecorrected molding surface 503 and the plane 505 is 0.0125 μm as shown inFIG. 9, the above-mentioned target value is achieved. That is, themaximum amount to be removed by etching is 0.175 μm.

[0061] Then, obtaining the number of times of etching and the etchingamount of one etching so that the number of times of etching isminimized, since the minimum etching amount h when the maximum depth is0.175 μm is 0.025 μm, the etching amount is 0.1 μm for the firstetching, 0.05 μm for the second etching and 0.025 μm for the thirdetching.

[0062] As described above, the sample 100 of the high-NA reflective-typelens 1 is formed by use of the preliminary shape metal molds 20 and 30not having undergone etching yet, the shape correction amounts of thepreliminary shape metal molds 20 and 30 (only the metal mold 30 in thepresent embodiment) can be determined through measurement of the amountwhich corresponds to the surface shape of the sample 100, and correctioncorresponding thereto can be easily performed.

[0063]FIGS. 10A to 10F show a manner of correcting the drag preliminaryshape metal mold 30 by dry etching. A process of correcting the dragpreliminary shape metal mold 30 by etching will be concretely describedwith reference to FIG. 2 and these figures.

[0064] First, as shown in FIG. 10A, a resist 31 is applied to the dragpreliminary molding surface 300 of the drag preliminary shape metal mold30 (step S14). As the material of the resist 31, for example, in thecase of laser beam writing, AZ1500 of Hoechst Aktiengesellschaft isused, and in the case of electron beam writing, ZEP-520 of Nippon ZeonCo., Ltd. is used. Then, as shown in FIG. 10B, a latent image is scannedby a laser beam condensed by a condenser lens 32 of a laser beam writingdevice, and a resist pattern is drawn on the resist 31 (step S15). Inthe case of electron beam writing, an electron beam exposure device isused instead of the laser beam writing device. The resist pattern may bedrawn by a combination of a light source and a spatial light modulationelement.

[0065] Then, as shown in FIG. 10C, the resist 31 is developed (stepS16), and as shown in FIG. 10D, dry etching is performed on the dragpreliminary shape metal mold 30 (step S17). The amount of the firstetching is 0.1 μm as mentioned above. Then, as shown in FIG. 10E, theresist 31 is removed (step S18), and the first etching process isfinished.

[0066] Then, steps S14 to S18 are repeated to perform dry etching untilcorrection is completed (step S19). In the present embodiment, dryetching is performed to a depth of 0.05 μm in the second etching and toa depth of 0.025 μm in the third etching as mentioned above to completecorrection, and the drag 3 having a corrected drag molding surface 301is completed (see FIG. 10F). In the present embodiment, the copepreliminary shape metal mold 20 is used as the cope 2 as it is.

[0067] Thus, by correcting the shape of the surface of the dragpreliminary shape metal mold 30 by selectively etching the surface ofthe drag preliminary shape metal mold 30 in accordance with the shapecorrection amount determined through the prior measurement, the dragmolding surface 301 can be obtained, so that the cope 2 and the drag 3with high precision can be easily manufactured.

[0068] When the drag 3 is completed, as shown in FIG. 3, the high-NAreflective-type lens 1 is formed by use of the cope 2 and the drag 3.Then, a reflective coating is selectively applied to the surface of thelens 1 so that the lens 1 has a structure as shown in FIG. 1.

[0069] By thus correcting by etching the formed preliminary shape metalmolds 20 and 30 (in the present embodiment, of them, the dragpreliminary shape metal mold 30 having a plane) in accordance with theshape correction amount obtained by the measurement value, the cope 2and the drag 3 of high precision compared to that of the cope and thedrag manufactured when the correction is performed by the conventionalmethod using machining can be manufactured, and by forming the high-NAreflective-type lens 1 by use of the cope 2 and the drag 3, ahigh-precision and inexpensive high-NA reflective-type lens 1 can beprovided.

[0070] <2. Second Embodiment>

[0071] In the first embodiment, the shape correction amount is obtainedfrom the manufacture error of the sample 100 by measuring the shape ofthe sample 100. However, in the case of small-size lenses, measuring theoptical performance is often easier than measuring the shape itself, andis higher in practical utility.

[0072] First, like in the first embodiment, the sample 100 is formedfrom the cope preliminary shape metal mold 20 and the drag preliminaryshape metal mold 30, the manufacture error of the formed sample 100 isobtained by measuring the optical performance of the sample 100, and theshape correction amount is determined. As a method of obtaining themanufacture error by measuring the optical performance of the sample100, for example, a method is used that is shown in “Evaluation of LaserOptics from the Spot Image,” Yasuhiro TANAKA, Kogaku (Optics, Journal ofthe Optical Society of Japan), Vol. 22, No. 8 (August, 1993),pp.456-461.

[0073] After the manufacture error is obtained by the above-describedmethod, the shape correction amount is determined based on it, and thedrag preliminary shape metal mold 30 is corrected by etching shown inFIGS. 10A to 10F in the first embodiment to form the drag 3. After thecope 2 and the drag 3 are formed, using them, the high-NAreflective-type lens 1 is formed by the method shown in FIG. 4.

[0074] By thus determining the shape correction amount throughmeasurement of the optical performance of the sample 100, metal moldswith high precision like that of the first embodiment can be easilymanufactured, and by using them, a high-precision optical element can bemanufactured.

[0075] <3. Third Embodiment>

[0076] In the above-described embodiment, the shape correction amount tobe removed by etching when the metal mold is manufactured is obtained bymeasuring the sample 100 formed from the preliminary shape metal molds20 and 30. However, the shape correction amount may be obtained bydirectly measuring the surface shape of the preliminary shape metalmolds 20 and 30. In this case, since the object to be corrected ismeasured, a higher-precision metal mold can be manufactured.

[0077]FIG. 11 shows a laser interferometer 61 which is a device formeasuring the preliminary shape metal molds. In FIG. 11, a diffractionoptical element (zone plate 62) for a sphere measurement comprisesconcentric diffraction patterns designed so as to generate a wave fronthaving the same shape as the asphere (the cope molding surface 201 ofthe cope 2) to be measured. When the drag preliminary shape metal mold30 is measured, a zone plate designed so as to generate a wave fronthaving the same shape as the drag molding surface 301 is used. Thepreliminary shape metal molds 20 and 30 are prepared by a similar methodto that of the above-described embodiments.

[0078] First, the shapes of the cope preliminary molding surface 200 andthe drag preliminary molding surface 300 are measured with the laserinterferometer 61, and the shape errors of the surfaces are calculated.Then, the position of a point Y on the second reflecting surface 14corresponding to an arbitrary point X on the first reflecting surface 13is obtained by ray tracing with the calculator or computer 63 so thatcorrection amount of the shape errors of the cope preliminary moldingsurface 200 and the drag preliminary molding surface 300 are reflectedin the shape of the second reflecting surface 14. That is, the positionof the point Y on the second reflecting surface 14 is obtained suchthat, in the high-NA reflective-type lens 1 in FIG. 12, even if thepoint X on the first reflecting surface 13 includes an error, lightreflected at the point X and reaches the second aperture 12 (theinfluence of the error of the point X is canceled).

[0079] Then, the point corresponding to the point Y is obtained on themeasured drag preliminary molding surface 300, and the distance betweenthe points is the shape correction amount of the second reflectingsurface 14 at the point Y. By obtaining the shape correction amount bythe above-described method for all the points on the first reflectingsurface 13, the overall shape correction amount of the second reflectingsurface 14 can be obtained. When the overall shape correction amount ofthe second reflecting surface 14 is obtained, the shape correctionamount of the drag preliminary molding surface 300 for molding thesecond reflecting surface 14 is obtained based on it.

[0080] When the shape correction amount of the drag preliminary moldingsurface 300 is obtained, etching is performed by a similar method tothat of the first embodiment to correct the drag preliminary moldingsurface 300, so that the cope 2 and the drag 3 are obtained. By usingthe cope 2 and the drag 3, the high-NA reflective-type lens 1 ismanufactured (see FIG. 4).

[0081] As described above, by using the laser interferometer 61 and thezone plate 62, the shape correction amount is determined throughmeasurement of the surface shapes of the preliminary shape metal molds20 and 30 not having undergone etching yet, and the prototypes of thecope 2 and the drag 3 to be corrected are directly measured, so that ahigher-precision metal mold can be manufactured.

[0082] <4. Fourth Embodiment>

[0083] While examples manufacturing optical elements with metal moldsare described in the above-described embodiments, when elements andparts not more than 1 mm are manufactured, lithography being inexpensiveand with which micromachining is possible is generally used.

[0084]FIG. 13 shows a microlens 7 which is an optical element in afourth embodiment. Although having a first aperture 71, a secondaperture 72, a first reflecting surface 73 and a second reflectingsurface 74 like the high-NA reflective-type lens 1 in theabove-described embodiments, the microlens 7 is different from the lens1 in that it is approximately 1 mm in diameter and smaller than the lens1.

[0085]FIG. 14 is a flowchart showing a process of manufacturing apreform of the microlens 7 (hereinafter, abbreviated as preform) byreflow which is a method of lithography. FIGS. 15A to 15D show a mannerof manufacturing the preform by reflow. With reference to these figures,a method of manufacturing the preform will be described.

[0086] First, as shown in FIG. 15A, a resist 76 is applied to a glassplate 75 which is the base material of the preform (step S21). Then, asshown in FIG. 15B, resist patterns are formed on the surface of theglass plate 75 by an exposure and development process (step S22).

[0087] After the resist patterns are formed, as shown in FIG. 15C, theyare heated to approximately 140° C. to soften the resist 76 so that theresist 76 is curved (step S23). Then, as shown in FIG. 15D, the shape ofthe resist 76 is transferred to the glass plate 75 by etching (step S24)to obtain a preform main body 77 a having the outside shape of themicrolens 7.

[0088] A reflective coating is applied to the preform main body 77 a(step S25) to form reflecting surfaces corresponding to the firstreflecting surface 73 and the second reflecting surface 74 shown in FIG.13. In this manner, a preform 77 having a shape corresponding to theoutside shape of the microlens 7 can be prepared.

[0089]FIG. 16 is a flowchart showing a process of manufacturing themicrolens 7 according to the fourth embodiment from the preform 77.FIGS. 17A to 17F show a manner of correcting the preform 77 by etchingto manufacture the microlens 7. With reference to these figures, amethod of manufacturing the microlens 7 will be described.

[0090] First, like in the first embodiment, the shape correction amountof the preform 77 is obtained by a measurement method using theMach-Zehnder interferometer 4 (step S31). The shape correction amountmay be obtained by the method shown in the second embodiment.

[0091] In this manner, the shape correction amount can be determinedthrough measurement of the surface shape of the preform of the opticalelement or measurement of the optical performance of the preform of theoptical element, so that a high-precision optical element can bemanufactured by the etching described later.

[0092] Here, description will be given on the assumption that themaximum correction amount δ is 0.2 μm like in the above-describedembodiments.

[0093] After the shape correction amount of the preform 77 is obtained,the reflective coating corresponding to the second reflecting surface 74of the microlens 7 is removed to return the preform 77 to the preformmain body 77 a (step S32), and as shown in FIG. 17A, a resist 78 isapplied to the surface on the substantially plane side of the performmain body 77 a (the surface corresponding to the second reflectingsurface 74 of the microlens 7) (step S33).

[0094] Then, as shown in FIG. 17B, a pattern is drawn on the resist 78by a condenser lens 79 of a laser beam writing device (step S34), and asshown in FIG. 17C, the resist 78 is developed (step S35).

[0095] Then, as shown in FIG. 17D, with the resist 78 as the mask, dryetching to a desired depth is performed on the preform main body 77 a(step S36), and as shown in FIG. 17E, the resist 78 is removed (stepS37). Steps S33 to S37 are repeated until correction is completed likein the first embodiment (step S38), and a reflective coating isselectively applied again onto the surface of the preform main body 77 a(step S39) to form the microlens 7 as shown in FIG. 17F.

[0096] By the above-described method, a small-size optical element likethe microlens 7 having a diameter of not more than 1 mm and on whichcorrection cannot be performed by the conventional method can beobtained by selectively etching the surface of the preform of theoptical element in accordance with the shape correction amountdetermined through prior measurement, so that a high-precision smalloptical element can be manufactured.

[0097] Moreover, by selectively etching only a specific optical surface,which is substantially plane, of a plurality of optical surfaces, aplurality of optical surfaces can be corrected all together, so that anoptical surface having a complicated shape can be corrected andcorrection can be performed more easily than when correction isperformed on all the optical surfaces.

[0098] <5. Modification>

[0099] While embodiments have been described, the present invention isnot limited to the above-described embodiments and various modificationsare possible.

[0100] For example, while dry etching is used as the method of etchingfor correction in the above-described embodiments, correction may beperformed by wet etching as long as the preliminary shape metal mold orthe optical element can be corrected in accordance with the obtainedshape correction amount.

[0101] While the high-NA reflective-type lens 1 is described as anexample of the object manufactured with the metal mold in the first tothe third embodiments, the object manufactured with the metal mold isnot limited to such an optical element. For example, the object may be asmall-size metal part, and may be anything that is generally molded witha metal mold.

[0102] While the preform of the microlens is prepared by use of reflowin the above-described fourth embodiment, other method can be employed.For example, isotropic etching or the like may be used, and any methodmay be used as long as the preform of the optical element ismanufactured with a predetermined precision.

[0103] As described above, by preparing a preliminary shape metal moldhaving a shape approximate to the outside shape of an object to bemolded and selectively removing the surficial part of the preliminaryshape metal mold by etching to obtain a molding surface, a metal moldwith a higher precision can be manufactured than when the moldingsurface is obtained by machining. In addition, manufacture efficiency ishigher than when a three-dimensional shape of an optical element or thelike is formed by repeating only lithography with a small processingamount a multiplicity of times.

[0104] Further, since the shape correction amount is determined throughmeasurement of the surface shape of the preliminary shape metal mold nothaving undergone etching yet, the object to be corrected is directlymeasured, so that a higher-precision metal mold can be manufactured.

[0105] Further, with the molding surface obtained by selectivelyremoving, by the shape correction amount, the surficial part of thepreliminary shape metal mold having a shape approximate to the outsideshape of the object to be molded by etching, a high-precision opticalelement and part can be effectively manufactured.

[0106] Further, the optical element as the object obtained by theabove-described method can be used for purposes requiring highprecision.

[0107] Further, by preparing a preform having a shape approximate to theoutside shape of an optical element and selectively etching thesurficial part of the preform to obtain the optical element, ahigh-precision optical element can be inexpensively manufactured.

[0108] Further, by correcting a plurality of optical surfaces alltogether by selectively etching a specific one of a plurality of opticalsurfaces, correction can be performed more easily than when correctionis performed on all the optical surfaces.

[0109] Further, by the substantially plane part being selected as thespecific optical surface, correction can be performed more precisely andeasily than when etching is performed on an optical surface having acomplicated shape.

[0110] Further, by the above-described method, an optical element with adiameter of not more than 1 mm can be manufactured with high precision.

[0111] Further, by providing an optical surface obtained by selectivelyremoving, by etching, the surficial part of a preform molded into ashape approximate to a predetermined optical function, the opticalelement can be used for purposes requiring high precision.

[0112] Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modifications will be apparent to those skilledin the art. Therefore, unless otherwise such changes and modificationsdepart from the scope of the present invention, they should be construedas being included therein.

What is claimed is:
 1. A method of manufacturing a metal mold formolding an object, comprising: preparing a preliminary shape metal moldhaving a shape approximate to an outside shape of the object; and aprocessing to obtain a molding surface by selectively removing asurficial part of the preliminary shape metal mold by etching.
 2. Amethod according to claim 1, wherein said processing includes:correcting a shape of a surface of the preliminary shape metal mold byselectively etching the surface of the preliminary shape metal mold inaccordance with a shape correction amount determined through priormeasurement.
 3. A method according to claim 2, wherein the shapecorrection amount is determined through measurement of a surface shapeof the preliminary shape metal mold not having undergone etching yet. 4.A method according to claim 2, wherein said processing includes: forminga sample of the object by use of the preliminary shape metal mold nothaving undergone etching yet; and determining the shape correctionamount through measurement of an amount in which a surface shape of thesample is reflected.
 5. A method according to claim 2, wherein saidprocessing includes: forming a sample of the object by use of thepreliminary shape metal mold not having undergone etching yet; anddetermining the shape correction amount through measurement of opticalperformance of the sample.
 6. A metal mold for molding an object,comprising: a first portion of molding surface obtained by a preliminarymanufacturing stage; and a second portion of molding surface obtained byselectively removing a surficial part obtained by the preliminarymanufacturing stage by etching in accordance with a predetermined shapecorrection amount.
 7. A metal mold according to claim 6, wherein theshape correction amount is determined through prior measurement.
 8. Ametal mold according to claim 7, wherein the shape correction amount isdetermined through measurement of shape of a surface obtained by thepreliminary manufacturing not having undergone etching yet.
 9. A metalmold according to claim 7, wherein a sample of the object is formed byuse of the mold obtained by the preliminary manufacturing not havingundergone etching yet and the shape correction amount is determinedthrough measurement of an amount in which a surface shape of the sampleis reflected.
 10. A metal mold according to claim 7, wherein a sample ofthe object is formed by use of the mold obtained by the preliminarymanufacturing not having undergone etching yet and the shape correctionamount is determined through measurement of optical performance of thesample.
 11. A method of manufacturing an optical element, comprising:preparing a preform having a shape approximate to an outside shape ofthe optical element; and processing for obtaining the optical element byselectively etching a surface shape of the preform.
 12. A methodaccording to claim 11, wherein said processing includes: selectivelyetching a surface of the preform in accordance with a shape correctionamount determined through prior measurement.
 13. A method according toclaim 12, wherein said processing further includes: determining theshape correction amount through measurement of an amount in which thesurface shape of the preform is reflected.
 14. A method according toclaim 12, wherein said processing further includes: determining theshape correction amount through measurement of optical performance ofthe preform.
 13. A method according to claims 11, wherein the opticalelement has a plurality of optical surfaces successively acting onincident light, and in the etching, the optical surfaces are correctedall together by selectively etching a specific one of the opticalsurfaces.
 14. A method according to claim 13, wherein the opticalsurfaces include a substantially plane part, and the substantially planepart is selected as the specific optical surface.
 15. A method accordingto claim 11, wherein the optical element is not more than 1 mm indiameter.